Process for the preparation of radiation image storage panels

ABSTRACT

A process for preparing a radiation image storage panel having a support and a stimulable phosphor layer comprises the first step of forming a phosphor sheet comprising a binder and a stimulable phosphor and the second step of compressing the phosphor sheet on the support under heating up to a temperature of not lower than softening point or melting point of the binder, using a plurality of calender rolls giving different pressure, a combination of a calender roll and a preheating means, or a combination of a calender roll and a tension-applying means.

This is a Divisional application of Ser. No. 08/196,460, filed Feb. 15,1994, which itself is a divisional application of Ser. No. 07/692,649,filed Apr. 29, 1991, now U.S. Pat. No. 5,306,367.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for preparing a radiation imagestorage panel which is employable in a radiation image recording andreproducing method utilizing a stimulable phosphor.

2. Description of the Prior Art

For obtaining a radiation image, there has been heretofore employed aradiography utilizing a radiographic film having a photosensitive silverhalide emulsion layer in combination with a radiographic intensifyingscreen.

As a method replacing the above-mentioned radiography, a radiation imagerecording and reproducing method utilizing a stimulable phosphor asdescribed, for example, in U.S. Pat. No. 4,239,968, has been utilized.In the radiation image recording and reproducing method, a radiationimage storage panel comprising a stimulable phosphor (stimulablephosphor sheet) is used, and the method involves steps of causing thestimulable phosphor of the panel to absorb radiation energy havingpassed through an object or having been radiated by an object; excitingthe stimulable phosphor with an electromagnetic wave such as visiblelight and infrared rays (namely, stimulating rays) to sequentiallyrelease the radiation energy stored in the stimulable phosphor as lightemission (stimulated emission); photo-electrically detecting the emittedlight to obtain electric signals; and reproducing the radiation image ofthe object as a visible image from the electric signals.

In the above-described radiation image recording and reproducing method,a radiation image is obtainable with a sufficient amount of informationby applying a radiation to the object at a considerably small dose, ascompared with the case of using the conventional radiography.Accordingly, this radiation image recording and reproducing method is ofgreat value especially when the method is used for medical diagnosis.

The radiation image storage panel employed in the radiation imagerecording and reproducing method has a basic structure comprising asupport and a stimulable phosphor-containing resin layer provided on onesurface of the support. Further, a transparent film is generallyprovided on the free surface (surface not facing the support) of thestimulable phosphor-containing resin layer to keep the stimulablephosphor-containing resin layer from chemical deterioration or physicalshock.

The stimulable phosphor-containing resin layer generally comprises aresinous binder and stimulable phosphor particles dispersed therein. Thestimulable phosphor particles in the stimulable phosphor-containingresin layer, when excited with stimulating rays after having beenexposed to a radiation such as X-rays, emit light (stimulated emission).Accordingly, the radiation having passed through an object or havingbeen radiated by an object is absorbed by the stimulablephosphor-containing resin layer of the radiation image storage panel inproportion to the applied radiation dose, and a radiation image of theobject is produced in the radiation image storage panel in the form of aradiation energy-stored image (in the form of a latent image). Theradiation energy-stored image can be released as stimulated emission (inthe form of light emission) by applying stimulating rays to the panel,for instance by scanning the panel with stimulating rays. The stimulatedemission is then photo-electrically converted to electric signals, so asto produce a visible image from the radiation energy-stored image.

Accordingly, it is desired for the radiation image storage panelemployed in the radiation image recording and reproducing method to havea high sensitivity and to provide an image of high quality (highsharpness, high graininess, etc.). In particular, from the viewpoint ofobtaining more accurate and detailed information of an object, it isdesired to develop a radiation image storage panel which provide animage of improved sharpness.

The sensitivity of the radiation image storage panel is essentiallydetermined by the total amount of stimulated emission produced by thestimulable phosphor contained therein, and the total amount variesdepending upon not only the emission luminance of the phosphor but alsothe content (i.e., amount) of the phosphor in the phosphor layer. Thelarge content of the phosphor also results in increase of absorption ofa radiation such as X-rays, so that the panel shows an increased highsensitivity and provides an image of improved quality, especially,improved graininess. On the other hand, assuming that the content of thephosphor layer is kept at the same level, a panel utilizing such aphosphor layer provides an image of high sharpness, if the phosphorlayer is densely packed with the phosphor, because such phosphor layercan be made thinner to reduce spread of stimulating rays which is causedby scattering in the phosphor layer.

U.S. Pat. No. 4,910,407 describes a radiation image storage panel havinga phosphor layer in which stimulable phosphor particles are denselypacked by compressing the prepared phosphor layer so as to reduce thevoid volume of the layer.

The phosphor density in the phosphor layer of the above-mentioned panelis increased by compressing the phosphor layer, and an image provided bythe panel exhibits improved sharpness as compared with one obtained by apreviously known radiation image storage panel. However, with respect tothe graininess, the panels sometimes relatively deteriorate.

U.S. patent application Ser. No. 07/510,679, now U.S. Pat. No. 5,164,224describes a radiation image storage panel showing high sharpness as wellas good graininess. The radiation image storage panel utilizes athermoplastic elastomer as a binder of the stimulable phosphor layer andis prepared by compressing a phosphor layer on a support at atemperature of not lower than softening point or melting point of thethermoplastic elastomer.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a processfor preparing a radiation image storage panel which is further improvedin the sharpness as well as graininess of the image provided thereby.

There is provided by the present invention a process (1) for thepreparation of a radiation image storage panel comprising the steps of:

forming a phosphor sheet comprising a binder and a stimulable phosphor;and

compressing the phosphor sheet on a support under heating up to atemperature of not lower than softening point or melting point of thebinder, at least twice, by means of a calender roll first at a lowpressure and second at a high pressure, so as to fix the phosphor sheetonto the support.

There is also provided by the invention a process (2) for thepreparation of a radiation image storage panel comprising the steps of:

forming a phosphor sheet comprising a binder and a stimulable phosphor;

heating the phosphor sheet on a support; and

compressing the heated phosphor sheet on the support under heating ormaintaining the heated phosphor sheet at a temperature of not lower thansoftening point or melting point of the binder, by means of a calenderroll, so as to fix the phosphor sheet onto the support.

There is further provided by the invention a process (3) for thepreparation of a radiation image storage panel comprising the steps of:

forming a phosphor sheet comprising a binder and a stimulable phosphor;and

compressing the phosphor sheet on a support under heating up to atemperature of not lower than softening point or melting point of thebinder by means of a heated calender roll, keeping the phosphor sheetunder tension.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a heating-compression system comprisingtwo sets of calender rolls which is employable for performing thecompression step of the process (1) for the preparation of a radiationimage storage panel according to the present invention.

FIG. 2 shows a schematic view of a known heating-compression systemcomprising one set of calender rolls which is generally employed forperforming a compression step.

FIGS. 3 to 6 show schematic views of heating-compression systemscomprising one or more sets of calender rolls and preheating means whichare employable for performing the compression step of the process (2)for the preparation of a radiation image storage panel according to thepresent invention.

FIG. 7 shows a schematic view of a heating-compression system comprisinga set of calender rolls and means for applying tension to the phosphorsheet which is employable for performing the compression step of theprocess (3) for the preparation of a radiation image storage panelaccording to the present invention.

FIG. 8 shows a schematic view of a known heating-compression systemcomprising one set of calender rolls but not being equipped with meansfor applying tension to the phosphor sheet which is generally employedfor performing a compression step.

FIGS. 9 to 11 graphically show relationships between graininess andsharpness of radiation images given by the radiation image storagepanels which are described in the working examples.

DETAILED DESCRIPTION OF THE INVENTION

In the processes of the invention, the phosphor sheet formed for thelamination on a sheet is compressed and affixed simultaneously on thesupport at a temperature not lower than softening point or melting pointof the binder of the phosphor layer. The crystalline phosphor dispersedin the binder of the phosphor sheet can move somewhat freely within thephosphor sheet during the compression treatment. Therefore, the pressureapplied onto the phosphor sheet further serves to extend the sheetgradually on the support to give a relatively thin phosphor layer evenif a relatively high pressure is applied.

Consequently, by utilizing the processes of the invention, void volumeof a phosphor layer of a radiation image storage panel is loweredwithout destruction of the phosphor, and a phosphor layer which has arelatively thin thickness and accordingly is able to give a radiationimage with enhanced sharpness is easily obtained.

The processes (1), (2) and (3) for the preparation of a radiation imagestorage panel according to the present invention all comprise two steps,namely, the first step for the preparation of a phosphor sheet, and thesecond step (or further the third step)for fixing the phosphor sheetonto a support by compressing the phosphor sheet with heating underdifferent conditions.

The processes of the invention are described in more detail below.

The first step is for forming a phosphor sheet comprising a binder and astimulable phosphor.

The phosphor layer of the radiation image storage panel comprises aresinous binder and stimulable phosphor particles dispersed therein.

The stimulable phosphor, as described hereinbefore, gives stimulatedemission when excited by stimulating rays after exposure to a radiation.In the viewpoint of practical use, the stimulable phosphor is desired togive stimulated emission when excited by stimulating rays in thewavelength region of 400-850 nm.

Examples of the stimulable phosphor employable in the radiation imagestorage panel of the present invention include:

SrS:Ce;Sm; SrS:Eu,Sm; ThO₂ :Er, and La₂ O₂ S:Eu,Sm; as described in U.S.Pat. No. 3,859,527;

ZnS:Cu,Pb; BaO·xAl₂ O₃ :Eu, in which x is a number satisfying thecondition of 0.8≦x≦10, and M²⁺ O·xSiO₂ :A, in which M²⁺ is at least onedivalent metal selected from the group consisting of Mg, Ca, St, Zn, Cdand Ba, A is at least one element selected from the group consisting ofCe, Tb, Eu, Tin, Pb, Tl, Bi and Mn, and x is a number satisfying thecondition of 0.5≦x≦2.5, as described in U.S. Pat. No. 4,326,078;

(Ba_(1-x-y),Mg_(x),Ca_(y))FX:aEu²⁺, in which X is at least one elementselected from the group consisting of Cl and Br, x and y are numberssatisfying the conditions of 00<x+y≦0.6, and xy≈0, and a is a numbersatisfying the condition of 10⁻⁶ ≦a≦5×10⁻², as described in JapanesePatent Provisional Publication No. 55(1980)-12143;

LnOX:xA, in which Ln is at least one element selected from the groupconsisting of La, Y, Gd and Lu, X is at least one element selected fromthe group consisting of Cl and Br, A is at least one element selectedfrom the group consisting of Ce and Tb, and x is a number satisfying thecondition of 0<x<0.1, as described in the above-mentioned U.S. Pat. No.4,236,078;

(Ba_(l-x),M^(II) _(x))FX:yA, in which M^(II) is at least one divalentmetal selected from the group consisting of Mg, Ca, St, Zn and Cd, X isat least one element selected from the group consisting of Cl, Br and I,A is at least one element selected from the group consisting of Eu, Tb,Ce, Tm, Dy, Pr, Ho, Md, Yb and Er, and x and y are numbers satisfyingthe conditions of 0≦x≦0.6 and 0≦y≦0.2, respectively, as described inJapanese Patent Provisional Publication No. 55(1980)-12145;

M^(II) FX·xA:yLn, in which M^(II) is at least one element selected fromthe group consisting of Ba, Ca, St, Mg, Zn and Cd; A is at least onecompound selected from the group consisting of BeO, MgO, CaO, SrO, BaO,ZnO, Al₂ O₃, Y₂ O₃, La₂ O₃, In₂ O₃, SiO₂, TiO₂, ZrO₂, GeO₂, SnO₂, Nb₂O₅, Ta₂ O₅ and ThO₂ ; Ln is at least one element selected from the groupconsisting of Eu, Tb, Ce, Tin, Dy, Pr, Ho, Nd, Yb, Er, Sm and Gd; X isat least one element selected from the group consisting of Cl, Br and I;and x and y are numbers satisfying the conditions of 5×10⁻⁵ ≦x≦0.5 and0<y≦0.2, respectively, as described in Japanese Patent ProvisionalPublication No. 55(1980)-160078;

(Ba_(1-x),M^(II) _(x))F₂ ·aBaX₂ :yEu,zA, in which M^(II) is at least oneelement selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd;X is at least one element selected from the group consisting of Cl, Brand I; A is at least one element selected from the group consisting ofZr and Sc; and a, x, y and z are numbers satisfying the conditions of0.5≦a≦1.25, 0≦x≦1, 10⁻⁶ ≦y≦2×10⁻¹, and 0<z≦10⁻² respectively asdescribed in Japanese Patent Provisional Publication No.56(1981)-116777;

(Ba_(1-x),M^(II) _(x))F₂ ·aBaX₂ :yEu,zB, in which M^(II) is at least oneelement selected from the group consisting of Be, Mg, Ca, St, Zn and Cd;X is at least one element selected from the group consisting of Cl, Brand I; and a, x, y and z are numbers satisfying the conditions of0.5≦a≦1.25, 0≦x≦1, 10⁻⁶ ≦y≦2×10⁻¹, and 0<z≦2×10⁻¹, respectively asdescribed in Japanese Patent Provisional Publication No. 57(1982)-23673;

(Ba_(1-x),M^(II) _(x))F₂ ·aBaX₂ :yEu,zA, in which M^(II) is at least oneelement selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd;X is at least one element selected from the group consisting of Cl, Brand I; A is at least one element selected from the group consisting ofAs and Si; and a, x, y and z are numbers satisfying the conditions of0.5≦a≦1.25, 0≦x≦1, 10⁻⁶ ≦y≦2×10⁻¹, and 0<z≦5×10⁻¹, respectively, asdescribed in Japanese Patent Provisional Publication No. 57(1982)-23675;

M^(III) OX:xCe, in which M^(III) is at least one trivalent metalselected from the group consisting of Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho,Er, Tm, Yb, and Bi; X is at least one element selected from the groupconsisting of Cl and Br; and x is a number satisfying the condition of0<x<0.1, as described in Japanese Patent Application No. 56(1981)-167498;

Ba_(1-x) M_(x/2) L_(x/2) FX:yEu²⁺ in which M is at least one alkalimetal selected from the group consisting of Li, Na, K, Rb and Cs; L isat least one trivalent metal selected from the group consisting of Sc,Y, La, Ce, Pr, Nd, Pro, Sin, Gd, Tb, Dy, Ho, Er, Tin, Yb, Lu, Al, Ga, Inand Tl; X is at least one halogen selected from the group consisting ofCl, Br and I; and x and y are numbers satisfying the conditions of 10⁻²≦x≦0.5 and 0<y≦0.1, respectively;

BaFX·xA:yEu²⁺, in which X is at least one halogen selected from thegroup consisting of Cl, Br and I; A is at least one fired product of atetrafluoroboric acid compound: and x and y are numbers satisfying theconditions of 10⁻⁶ ≦x≦0.1 and 0<y≦0.1, respectively;

BaFX·xA:yEu²⁺, in which X is at least one halogen selected from thegroup consisting of Cl, Br and I; A is at least one fired product of ahexafluoro compound selected from the group consisting of monovalent anddivalent metal salts of hexafluoro silicic acid, hexafluoro titanic acidand hexafluoro zirconic acid; and x and y are numbers satisfying theconditions of 10⁻⁶ ≦x≦0.1 and 0<y≦0.1, respectively;

BaFX·xNaX':aEu²⁺, in which each of X and X' is at least one halogenselected from the group consisting of Cl, Br and I; and x and a arenumbers satisfying the conditions of 0<x≦2 and 0<a≦0.2, respectively;

M^(II) FX·xNaX':yEu²⁺ :zA, in which M^(II) is at least one alkalineearth metal selected from the group consisting of Ba, Sr and Ca; each ofX and X' is at least one halogen selected from the group consisting ofCl, Br and I; A is at least one transition metal selected from the groupconsisting of V, Cr, Mn, Fe, Co and Ni; and x, y and z are numberssatisfying the conditions of 0<x≦2, 0<y≦0.2 and 0<z≦10⁻² respectively;and

M^(II) FX·aM^(I) X'·bM'^(II) X"₂ √cM^(III) X"'₃ ·xA:yEu²⁺, in whichM^(II) is at least one alkaline earth metal selected from the groupconsisting of Ba, Sr and Ca; M^(I) is at least one alkali metal selectedfrom the group consisting of Li, Na, K, Rb and Cs; M'^(II) is at leastone divalent metal selected from the group consisting of Be and Mg;M^(III) is at least one trivalent metal selected from the groupconsisting of Al, Ga, In and Tl; A is at least one metal oxide; X is atleast one halogen selected from the group consisting of Cl, Br and I;each of X', X" and X"'is at least one halogen selected from the groupconsisting of F, Cl, Br and I; a, b and c are numbers satisfying theconditions of 0≦a≦2, 0≦b≦10⁻², 0≦c≦10⁻² and a+b+c≧10⁻⁶ ; and x and y arenumbers satisfying the conditions of 0<x≦0.5 and 0<y≦0.2, respectively.

The above-described stimulable phosphors are given by no means torestrict the stimulable phosphor employable in the present invention.Any other phosphors can be also employed, provided that the phosphorgives stimulated emission when excited with stimulating Pays afterexposure to a radiation.

The stimulable phosphor in a powder form is well mixed with a resinousbinder in an appropriate solvent to give a coating dispersion whichcontains stimulable phosphor particles uniformly dispersed in a bindersolution.

Preferred examples of the resinous binder to be used for the preparationof the phosphor sheet include thermoplastic elastomers which haveelasticity at room temperatures and show flowability when they areheated, such as polystyrene, polyolefin, polyurethane, polyester,polyamide, polybutadiene, ethylene-vinyl acetate copolymer, polyvinylchloride, natural rubber, fluorocarbon rubber, polyisoprene, chlorinatedpolyethylene, styrene-butadiene rubber, and silicone rubber.

Preferably, the thermoplastic elastomers have softening or meltingpoints in the range of 30° C. to 300° C. More preferably, thethermoplastic elastomers have softening or melting points in the rangeof 30° C. to 200° C. Most preferably, the thermoplastic elastomers havesoftening or melting points in the range of 30° C. to 150° C.

Examples of the solvents employable in the preparation of the coatingdispersion include lower alcohols such as methanol, ethanol, n-propanoland n-butanol; chlorinated hydrocarbons such as methylene chloride andethylene chloride; ketones such as acetone, methyl ethyl ketone andmethyl isobutyl ketone; esters of lower alcohols with lower aliphaticacids such as methyl acetate, ethyl acetate and butyl acetate; etherssuch as dioxane, ethylene glycol monoethylether and ethylene glycolmonoethyl ether; and mixtures of the above-mentioned compounds.

The ratio between the binder and the phosphor in the coating dispersionmay be determined according to the characteristics of the aimedradiation image storage panel and natures of the phosphor and binderemployed. Generally, the ratio is within the range of from 1:1 to 1:100(binder:phosphor, by weight), preferably from 1:8 to 1:40.

The coating dispersion may contain a dispersing agent to assistdispersibility of the phosphor particles therein, and also may contain avariety of additives such as a plasticizer for increasing the bondingbetween the binder and the phosphor particles in the phosphor layer.Examples of the dispersing agent include phthalic acid, stearic acid,caproic acid and a hydrophobic surface active agent. Examples of theplasticizer include phosphates such as triphenyl phosphate, tricresylphosphate and diphenyl phosphate; phthalates such as diethyl phthalateand dimethoxyethyl phthalate; glycolates such as ethylphthalyl ethylglycolate and butylphthalyl butyl glycolate; and polyesters ofpolyethylene glycols with aliphatic dicarboxylic acids such as polyesterof triethylene glycol with adipic acid and polyester of diethyleneglycol with succinic acid.

The coating dispersion containing the phosphor particles and binderprepared is then applied uniformly onto a surface of a false support(temporary support) to form a layer of the coating dispersion. Thecoating procedure can be carried out by a conventional method such as amethod using a doctor blade, a roll coater or a knife coater.

The false support can be selected from the support materials known forradiation image storage panels. Examples of the support materialsinclude plastic films such as films of cellulose acetate, polyester,polyethylene terephthalate, polyamide, polyimide, triacetate andpolycarbonate; metal sheets such as aluminum foil and aluminum alloyfoil; ordinary papers; baryta paper; resin-coated papers; pigment paperscontaining titanium dioxide or the like; and papers sized with polyvinylalcohol or the like. The false support also can be prepared by a sheetof ceramics such as alumina, titania, zirconia or magnesia, a glasssheet, or a metal sheet. Preferably, the false sheet has a releasinglayer on the surface so that the formed phosphor sheet can be readilypeeled off.

After applying the coating dispersion to the false support, the coatingdispersion is then heated slowly to dryness so as to complete theformation of a phosphor sheet. Thickness of the phosphor sheet variesdepending upon the characteristics of the aimed radiation image storagepanel, the nature of the phosphor, the ratio between the binder and thephosphor, etc. Generally, the thickness of the phosphor layer is withina range of from 20 μm to 1 mm, preferably from 50 to 500 μm.

Subsequently thus formed phosphor sheet is fixed on a genuine support byapplying pressure and heat to the phosphor sheet in the second step.

The material of the genuine support can be the same as that of the falsesupport. However, a plastic film is preferably employed as the supportmaterial. The plastic film may contain a light-absorbing material suchas carbon black, or may contain a light-reflecting material such astitanium dioxide.

In the preparation of a known radiation image storage panel, one or moreadditional layers are occasionally provided between the support and thephosphor layer so as to enhance the adhesion between the support and thephosphor layer, or to improve sensitivity of the panel or the quality ofan image provided thereby. For instance, a subbing layer or an adhesivelayer may be provided by coating polymer material such as gelatin overthe surface of the support on the phosphor layer side. Otherwise, alight-reflecting layer or a light-absorbing layer may be provided byforming a polymer material layer containing a light-reflecting materialsuch as titanium dioxide or a light-absorbing material such as carbonblack. In the invention, one or more of these additional layers may beprovided depending on the type of the radiation image storage panel.

The phosphor layer side surface of the support (or the surface ofadhesive layer, light-reflecting layer, or light-absorbing layer in thecase where such layers provided on the phosphor layer) may be providedwith protruded and depressed portions for enhancement of the sharpnessof radiation image, and the constitution of those protruded anddepressed portions can be selected depending on the purpose of theradiation image storage panel.

The second step of the process (1) is now described by referring to FIG.1 of the attached drawings.

The phosphor sheet prepared in the first step (1) is placed on a supportand compressed under heating, by means of a calender roll first at a lowpressure and second at a high pressure. The compression under heating isgenerally performed using at least two sets of calender rolls, namely, afirst roll for application of a relatively low pressure and a secondroll for application of a relatively high pressure. By the compressionprocessing, the phosphor sheet is made thinner and fixed to the support.In addition to these sets of calender rolls, other calender rolls or anyother apparatus can be employed, if desired.

The conditions for the compression processing under heating can bevaried depending on nature of the binder, thickness of the phosphorsheet, etc., but are generally set to a pressure of 10-1,000 kg/cm² anda temperature of 30°-200° C. for the first roll, and to a pressure of50-2,000 kg/cm² and a temperature of 30°-200° C. for the second roll.Each of the first roll and second roll generally consists of two rollsas is illustrated in FIG. 1, and the temperature for the first andsecond rolls means an average temperature of these two rolls.

In the second step of the process (1), the pressure applied by the firstroll is lower than that applied by the second roll. The differencebetween the pressure of the first roll and that of the second rollpreferably ranges 10 to 1,000 kg/cm², more preferably ranges 50 to 500kg/cm². The temperature of the first roll generally is the same as orlower than the temperature of the second roll. But, the temperature ofthe first roll may be higher than the temperature of the second roll,provided that the difference of temperature does not exceed 30° C. Inany case, the highest temperature should exeed the softening or meltingtemperature of the resinous binder of the phosphor sheet.

The calender rolls can be selected from those used for the preparationof magnetic recording tapes or similar calender rolls. As is mentionedabove, the calender roll generally consists of two rolls. These tworolls may consist of two metal rolls, two rubber rolls, or a metal rolland a rubber roll. The combined structure of the phosphor sheet and thesupport is passed through between these two rolls at the predeterminedpressure.

The second and third steps of the process (2) are now described byreferring to FIGS. 3-6 of the attached drawings.

The phosphor sheet prepared in the first step is placed on a support andpre-heated, and then it is subjected to the compressing step.

In FIG. 3, a composite body 15 consisting of a phosphor sheet 13 and asupport 14 is preheated by means of a heater 12 and then compressed by aset of calender rolls 11A, 11B under heating or keeping the composite atthe temperature raised by the heater 12. By this compression processing,the phosphor sheet 13 is fixed on the support 14. Examples of the heaterinclude a far-infrared heater or a hot air heater. In FIG. 3, a set oftwo heaters are employed. However, the preheating can be done using onlyone heater which is preferably arranged on the phosphor sheet side.

For the compression processing, two sets of calender rolls can beemployed as is illustrated in FIG. 4. In FIG. 4, a composite body 25consisting of a phosphor sheet 23 and a support 24 is preheated by aheater 22 and then compressed by first calender rolls 21A, 21B underheating or keeping the heated condition. Thus compressed composite body25 is again heated by a heater 22' and then again compressed by secondcalender rolls 21'A, 21'B. By these processing, the phosphor sheet 23 isfirmly fixed onto the support 24. In addition to these heating means andcalender rolls, other calender rolls or any other apparata can beemployed, if desired.

FIG. 5 illustrates use of a set of heat rollers 32 as the heating meansfor heating a composite body 35 of a phosphor sheet 33 and a support 34.Thus heated composite body 35 is then compressed by means of a set ofcalender rolls 31A, 31B.

FIG. 6 illustrates use of two sets of heat rollers 42, 42' and two setsof calender rolls 41A, 41B, 41'A, 41'B. A composite body 45 of aphosphor sheet 43 and a support 44 is first heated by heater 42 andcompressed by calender rolls 41A, 41B, and again heated by heaters 42'and finally compressed by calender rolls 41'A, 41'B.

The preheating by heating means preferably raises the temperature of thephosphor sheet to not lower than 20° C. below the softening or meltingtemperature of the resinous binder but not higher than 50° C. above thesoftening or melting temperature of the binder.

The conditions for the compression processing are generally set to apressure of 50 kg/cm² or higher, where the compression processing isperformed once as is illustrated in FIGS. 3 and 5. In the case of usingtwo sets of calender rolls are used as is illustrated in FIGS. 4 and 6,the pressures for the compression are set to 10-1,000 kg/cm² for thefirst roll and 50-2,000 kg/cm², for the second roll.

Examples of the calender rolls are set forth in the description of thesecond step of the process (1).

The second step of the process (3) is now described by referring to FIG.7 of the attached drawings.

The phosphor sheet prepared in the first step is placed on a support andthen compressed under heating up to a temperature of not lower thansoftening point or point of the binder by means of a heated calenderroll, keeping the phosphor sheet under tension.

In FIG. 7, a calender roll 51 comprises a roll 51a (upper roll) and aroll 51b (lower roll) and further has a torque roll 52 which serves togive a tension to a phosphor sheet. Around the torque roll 52, aphosphor sheet 53 which was prepared in the first step is wound. Due topredetermined difference of rotation rate, rotation timing between thetorque roll 52 and the calender roll 51, a desired tension (weight) isgiven to the phosphor sheet 53. The tension or weight given to thephosphor sheet 53 can be detected by means of a tension detector such asa tension pick-up roll 54. Thus detected tension value is transmitted tothe calender roll 51 and/or the torque roIl 12 to control the rotationconditions of these rolls.

According to FIG. 7, a phosphor sheet 53 and a support 54 is combinedjust before these are introduced into the calender roll 51. Examples ofthe calender roll are described hereinbefore.

The conditions for the compression processing using the caIender rollare generally set to a pressure of 50-2,000 kg/cm² and to a temperatureof 80°-200° C.

The tension (or weight) applied to the phosphor sheet is generally soset as to give an elongation ratio in the range of 1-150% (preferably5-50%). This means that the length of the untreated phosphor sheet isprolonged to a length as much as 1.01 to 2.5 times (preferably 1.05 to1.5 times) when it is processed by the compression under heating andtension. For giving such elongation condition, the tension applied tothe phosphor sheet is preferably adjusted to a value in the range of10-700 g/cm (preferably 20-500 g/cm). However, the preferred tensionranges may vary depending on nature and thickness of the phosphor sheet.

The void volume of the stimulable phosphor-containing resin layer (i.e.,phosphor layer) formed on the support in the above-described manners canbe calculated theoretically by the following formula (I), ##EQU1## inwhich V is a total volume of the phosphor layer; Vair is a volume of aircontained in the phosphor layer; A is a total weight of the phosphor; ρxis a density of the phosphor; ρy is a density of the binder; ρair is adensity of air; a is a weight of the phosphor; and b is a weight of thebinder.

In the formula (I), ρair is nearly 0. Accordingly, the formula (I) canbe approximately rewritten in the form of the following formula (II):##EQU2## in which V, Vair, A, ρx, ρy, a and b have the same meanings asdefined in the formula (I).

The void volume of the phosphor layer is expressed by a value calculatedaccording to the formula (II).

The radiation image storage panel generally has a transparent film on afree surface of a phosphor layer to protect the phosphor layer fromphysical and chemical deterioration. In the radiation image storagepanel of the present invention, it is preferable to provide atransparent film for the same purpose.

The transparent film can be provided onto the phosphor layer by coatingthe surface of the phosphor layer with a solution of a transparentpolymer such as a cellulose derivative (e.g. cellulose acetate ornitrocellulose), or a synthetic polymer (e.g. polymethyl methacrylate,polyvinyl butyral, polyvinyl formal, polycarbonate, polyvinyl acetate,or vinyl chloride-vinyl acetate copolymer), and drying the coatedsolution. Alternatively, the transparent film can be provided onto thephosphor layer by beforehand preparing it from a polymer such aspolyethylene terephthalate, polyethylene, polyvinylidene chloride orpolyamide, followed by placing and fixing it onto the phosphor layerwith an appropriate adhesive agent. The transparent protective filmpreferably has a thickness within the range of approx. 0.1 to 20 μm.

The following examples further illustrate the present invention, butthese examples are by no means understood to restrict the invention.

EXAMPLE 1

In a mixture solvent of methyl ethyl ketone and 2-propanol (1:1), 200 gof a particulate divalent europium activated barium fluorobromoiodidestimulable phosphor (BaFBr₀.9 I₀.1 :Eu²⁺), 22.5 g of a resinous binder(polyurethane elastomer, Desmolack TPKL-5-2625 (solid content: 40%),available from Sumitomo Byer Urethane Co., Ltd.) and 1.0 g of ananti-yellowing agent (epoxy resin, Epicoat 1007, available from YukaShell Epoxy Co., Ltd.) were dispersed using a propeller agitator to givea coating composition (binder/phosphor=1/20) having a viscosity of 30 PS(at 25° C.). The coating composition was coated on a polyethyleneterephthalate sheet (false support, thickness: 180 μm) having asilicone-type release agent layer thereon. The coated composition wasdried and peeled off from the false support to give a phosphor sheet.The vicar softening temperature of the binder was 45° C. (ASTM D1525).

Independently, a coating composition for the formation of alight-reflecting layer was prepared by dispersing 214 g of BaFBr (90% ofparticles had particle sizes within the range of 1-5 μm), 25.7 g (assolid content) of soft acrylic resin, 10.7 g of epoxy resin and 64 g ofnitrocellulose (nitration degree: 11.5%, solid content: 10 wt. %) inmethyl ethyl ketone by a propeller agitator to give a coatingcomposition having a viscosity of 25-35 PS (at 25° C.).

Further, a coating composition for the formation of a undercoating layerwas prepared by mixing 90 g (as solid content) of soft acrylic resin and50 g of nitrocellulose in methyl ethyl ketone to give a coatingcomposition having a viscosity of 3-6 PS (at 25° C.).

The coating dispersion for undercoating layer was uniformly coated overa polyethylene terephthalate sheet (genuine support, thickness; 300 μm)placed horizontally on a glass plate. The coating procedure was carriedout using a doctor blade. The support having the coated layer was thenheated to a temperature gradually rising from 25° to 100° C. Thus, asheet consisting of a support and a undercoating layer (thickness: 15μm) was made. Over the undercoating layer of the prepared sheet wascoated and heated the coating composition for light-reflecting layer inthe same manner as above to form a light-reflecting layer (thickness: 60μm) on the sheet. Thus, a support sheet having the undercoating layerand light-reflecting layer was prepared.

On the support sheet was placed the phosphor sheet, and the resultingcomposite body was compressed by means of two sets of calender rolls(first roll and second roll) made of iron as illustrated in FIG. 1,under the following conditions.

    ______________________________________                                                      First Roll                                                                            Second Roll                                             ______________________________________                                        Roll Temperature                                                              (Upper Roll)    50° C.                                                                           90° C.                                       (Lower Roll)    50° C.                                                                           90° C.                                       Pressure applied                                                                              100 kg/cm.sup.2                                                                         500 kg/cm.sup.2                                     Feed Rate       0.2 m/min.                                                                              0.2 m/min.                                          ______________________________________                                    

By the compression processing, the phosphor sheet was firmly fixed onthe light-reflecting layer of the support.

On the phosphor layer was placed a transparent polyethyleneterephthalate film (thickness: 10 μm; provided with a polyesterandhesive layer) to combine the transparent film and the phosphor layerthrough the adhesive layer.

Thus, a radiation image storage panel having the phosphor layer on asupport was prepared.

Comparison Example 1

The procedure of Example 1 was repeated except that the compressionprocessing was performed using one set of calender rolls made of iron asshown in FIG. 2 under the conditions of temperatures of upper and lowerrolls at 70° C., a pressure of 600 kg/cm² and a feed rate of 0.2 m/min.

Thus, a radiation image storage panel having the phosphor layer on asupport was prepared.

In the compression processing, it was observed that the phosphor sheetsometimes adhered to the surface of the upper roll.

Comparison Example 2

The procedure of Example 1 was repeated except that the compressionprocessing was performed using one set of calender rolls made of iron asshown in FIG. 2 (same as that of Comparison Example 1) under theconditions of temperatures of upper and lower rolls at 90° C., apressure of 600 kg/cm² and a feed rate of 0.2 m/min.

Thus, a radiation image storage panel having the phosphor layer on asupport was prepared.

In the compression processing, it was also observed that the phosphorsheet sometimes adhered to the surface of the upper roll.

EXAMPLE 2

The procedure of Example 1 was repeated using the same calender rollsexcept that the compression processing was performed under the followingconditions.

    ______________________________________                                                      First Roll                                                                            Second Roll                                             ______________________________________                                        Roll Temperature                                                              (Upper Roll)    70° C.                                                                           110° C.                                      (Lower Roll)    70° C.                                                                           110° C.                                      Pressure applied                                                                              100 kg/cm.sup.2                                                                         400 kg/cm.sup.2                                     Feed Rate       1 m/min.  1 m/min.                                            ______________________________________                                    

Thus, a radiation image storage panel having the phosphor layer on asupport was prepared.

Comparison Example 3

The procedure of Example 1 was repeated except that the compressionprocessing was performed using one set of calender rolls made of iron asshown in FIG. 2 under the conditions of temperatures of upper and lowerrolls at 80° C., a pressure of 500 kg/cm² and a feed rate of 1 m/min.

Thus, a radiation image storage panel having the phosphor layer on asupport was prepared.

In the compression processing, it was observed that the phosphor sheetsometimes adhered to the surface of the upper roll.

Evaluation of Radiation Image Storage Panel

The radiation image storage panels prepared as described above wereevaluated on the quality of the image according to the following test.

The radiation image storage panel was exposed to X-rays at voltage of 80KVp through an MTF chart and subsequently scanned with He-Ne laser beam(wavelength: 632.8 nm) to excite the phosphor. The light emitted by thephosphor layer was detected and converted to the corresponding electricSignals by means of a photosensor. The electric signals were reproducedby an image reproducing apparatus to obtain a visible image on arecording apparatus, and the modulation transfer function (MTF) value ofthe visible image was determined. The MTF value was given as a value (%)at the spacial frequency of 2 cycle/min. Further, a graininess at 0.1 mR(RMS) was determined. The results are graphically shown in FIG. 9.

In FIG. 9, the degree of sharpness (in terms of MTF value at spacialfrequency 2 cycle/mm) is plotted on the ordinate, wherein a larger valuemeans a higher sharpness. The degree of graininess is plotted on theabscissa, wherein larger value means a worse graininess.

As is apparent from FIG. 9, the radiation image storage panels preparedby the present invention give improved graininess, as compared with theradiation image storage panels prepared using one set of calender rollsunder similar heating condition, while the sharpness are keptessentially on the same level.

EXAMPLE 3

The procedure of Example 1 was repeated except that the compressionprocessing was performed using one set of calender rolls and afar-infrared heater as shown in FIG. 3 at a roll pressure of 600 kg/cm²and a feed rate of 2 m/min. The temperature of the phosphor sheet afterthe heating was measured by inserting a thermocouple (possiblemeasurement error: ±10° C.) between the phosphor sheet and the support.The temperature was 70° C. The compressed phosphor sheet had a thicknessof 170 μm.

Thus, a radiation image storage panel having the phosphor layer on asupport was prepared.

EXAMPLE 4

The procedure of Example 3 was repeated except that the roll pressurewas changed to 500 kg/cm² to give a compressed sheet of 185 μm, toprepare a radiation image storage panel having the phosphor layer on asupport.

Comparison Example 4

The procedure of Example 3 was repeated except that the far-infraredheaters 12 were swiched off but the lower roll 11B was heated to 70° C.,to prepare a radiation image storage panel having the phosphor layer ona support.

Comparison Example 5

The procedure of Example 4 was repeated except that the far-infraredheaters 12 were swiched off but the lower roll 11B was heated to 70° C.,to prepare a radiation image storage panel having the phosphor layer ona support.

Comparison Example 6

The procedure of Comparison Example 4 was repeated except that the feedrate was changed to 0.2 m/min., to prepare a radiation image storagepanel having the phosphor layer on a support.

Evaluation of Radiation Image Storage Panel

The radiation image storage panels prepared as described above wereevaluated on the quality of the image according to the same tests asdescribed above.

The results are graphically shown in FIG. 10.

As is apparent from FIG. 10, the radiation image storage panels preparedby the present invention give improved image quality, as compared withthe radiation image storage panels prepared with no preheating under thesame feeding conditions. Further, it is understood that the imagequality provided by the radiation image storage panel prepared by thepresent invention can be obtained by the process with no preheating onlywhen the feet rate is decreased to one tenth (1/10).

EXAMPLE 5

The procedure of Example 1 was repeated except that the compressionprocessing was performed using one set of calender rolls, a torque roll,and a tension pick-up roll as shown in FIG. 7 at a roll temperature of80° C. (same for the two calender rolls), a roll pressure of 500 kg/cm²,a feed rate of 1 m/min., and a tension (applied to the phosphor sheet)of 30 g/cm.

Thus, a radiation image storage panel having the phosphor layer on asupport was prepared.

EXAMPLE 6

The procedure of Example 5 was repeated except that the tension appliedto the phosphor was changed to 150 g/cm, to prepare a radiation imagestorage panel having the phosphor layer on a support.

EXAMPLE 7

The procedure of Example 5 was repeated except that the tension appliedto the phosphor was changed to 500 g/cm, to prepare a radiation imagestorage panel having the phosphor layer on a support.

Comparison Example 8

The procedure of Example 5 was repeated except that the calender roll 61(calender roll 61a, calender roll 61b) having no tension applying meanswas used. A phosphor sheet 63 was placed on a support 65 to pass throughbetween the two rolls to prepare a radiation image storage panel havingthe phosphor layer on a support.

Evaluation of Radiation Image Storage Panel

The radiation image storage panel prepared in the above examples showedthe following dimension changes.

    ______________________________________                                                    Ex. 5                                                                              Ex. 6    Ex. 7  Com. Ex. 8                                   ______________________________________                                        Tension (g/cm)                                                                               30    150      500   0                                         Thickness (μm)                                                             Before compression                                                                          300    400      500  270                                        After compression                                                                           200    210      195  200                                        Elongation ratio (%)                                                                         7      40       86   2                                         ______________________________________                                    

The radiation image storage panels prepared as

The radiation image storage panels prepared as described above wereevaluated on the quality of the image according to the same tests asdescribed above.

The results are graphically shown in FIG. 11.

As is apparent from FIG. 11, the radiation image storage panels preparedby the present invention give improved graininess, as compared with theradiation image storage panels prepared with no tension applied, whilethe sharpness is kept on the same level.

We claim:
 1. A process for the preparation of a radiation image storagepanel comprising the steps of:forming a phosphor sheet comprising abinder and a stimulable phosphor; and compressing the phosphor sheet ona support under heating up to a temperature of not lower than softeningpoint or melting point of the binder by means of a heated calender roll,keeping the phosphor sheet under tension in the range of 10-700 g/cm. 2.The process as defined in claim 1, wherein the binder is a thermoplasticelastomer.
 3. The process as defined in claim 1, wherein the tensionapplied to the phosphor sheet is in the range of 20 to 500 g/cm.
 4. Theprocess as defined in claim 1, wherein the tension is applied to thephosphor sheet to give an elongation ratio in the range of 1 to 150%. 5.The process as defined in claim 1, wherein the tension is applied to thephosphor sheet to give an elongation ratio in the range of 5 to 50%. 6.The process as defined in claim 1, wherein the binder is a thermoplasticelastomer and has a softening point of 30° to 200° C.